dt-bindings: clock: qcom,rpmcc: add clocks property

The RPM clock controller receive input clock ("xo").  It is modelled on
only one chip - MSM8953.

Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Link: https://lore.kernel.org/r/20220401201035.189106-11-krzysztof.kozlowski@linaro.org